(char? #\a)
=> #t
(char? #\Z)
=> #t
(char? #\newline)
=> #t
(char? #t)
=> #f
(char? #f)
=> #f
(char? ())
=> #f
(char? (char? #t))
=> #f
(char? 0)
=> #f
(char? 23870)
=> #f
(char? -23789)
=> #f